Process for the electrolytic production of manganese dioxide

ABSTRACT

ELECTROLYTIC PRODUCTION OF MANGANESE DIOXIDE FROM AN ELECTROLYTE WITH THE USE OF LEAD CATHODES. THE ELECTROLYTE SUBSTANTIALLY CONSISTS OF A COPPER-CONTAMINATED MANGANESE SULFATE SOLUTION IN SULFURIC ACID AND IS TREATED TO CONTAIN COPPER IN A CONCENTRATION OF LESS THAN 0.0005 WEIGHT PERCENT.

Jan. 1l, 1972 E. PREISLER l-TrAL PROCESS FOR THE ELEOTROLYTIC PRODUCTION OF MANGANES DIOXIDE Filed Feb. 16, 1970 m gama@ awww @www /uqesoJJog qd United States Patent U.s. Cl. 204-83 9 Claims ABSTRACT OF THE DISCLOSURE Electrolytic production of manganese dioxide from an electrolyte with the use of lead cathodes. The electrolyte substantially consists of a copper-contaminated manganese sulfate solution in sulfuric acid and is treated to contain copper in a concentration of less than 0.0005 weight percent.

The present invention relates to a process for the electrolytic Iproduction of manganese dioxide from an electrolyte with the use of lead cathodes, the electrolyte consisting substantially of a copper-contaminated manganese sulfate solution in sulfuric acid.

Lead cathodes are the cathodes normally used for the electrolytic production of manganese dioxide from a manganese sulfate solution in sulfuric acid. The reason for this is that lead and some of its alloys have good corrosion resistant properties in a sulfuric acid medium, even at elevated temperatures. These properties are occasioned by or correlated to the formation of a coherent protective lead sulfate film on the metallic cathode surface. vIf lead is the cathode material, the lead sulfate in fact undergoes cathodic reduction without formation of a lead sulfate film. Despite this, the cathode material is not subject to dissolution phenomena as a result of the high electrolytic excess voltage found to occur across lead cathodes traversed by current of satisfactory density.

Experience has shown, however, that the stability in the electrolyte is not always ensured and that the lead may be subject to dissolution phenomena, despite a satisfactory cathode load. While the dissolution rate is not high enough to effect the disappearance of the mechanical strength of cathodes within normal operation periods, the fact remains that enough lead penetrates into the electrolyte with the result that the product obtained by anodic precipitation then contains relatively high proportions of lead, which forbid its use in many fields of application.

It has now unexpectedly been found that the dissolution of lead cathodes is primarily occasioned by a layer of copper found to form thereon. This is all the more an unexpected result bearing in mind that the electrolytic excess voltage found to occur across a copper cathode is not substantially smaller than that found to occur across a lead cathode having dimensions the same as those of the copper cathode and traversed by current of identical density (cf. the ligure of the accompanying diagram, curves I and II). The polarization of the cathode effected by the electrolytic excess voltage, the value of which increases as the current density increases (with negative signs) has been found to reach a Value of -960 millivolts for the lead cathode and about 800 millivolts for the copper electrode, for a current density of l.2 amperes persquare decimeter. Lead, however, is subject to dissolution phenomena only in those cases in which the cathode potential is less negative than the lead corrosion potential which approaches a value of -520 millivolts in the electrolyte used.

3,634,215 Patented Jan. 11., 1972 It has been observed, however, that spongy copper is deposited on the cathode once the copper in the electrolyte commences to exceed a given concentration. As a result, the true cathode surface is considerably enlarged with respect to the geometric surface. This is accompanied by a reduction of the real current density and accordingly of the electrolytic excess voltage across the cathode with the result that the cathode potential is likely to drop to values more positive than the corrosion potential of lead, despite the high nominal current density (cf. curves III and IV). Lead which comes into contact with cathode areas not tightly covered with copper is accordingly subject to dissolution phenomena, the dissolution rate being the higher the stronger the deviation of the cathode potential from the corrosion potential, in positive direction.

It has also been found that the formation of spongy copper precipitate is occasioned by the presence of a certain minimum proportion of copper ions in the electrolyte.

The process of the present invention for the electrolytic production of manganese dioxide with the use of lead cathodes from a copper-contaminated electrolyte consisting substantially of a manganese sulfate solution in sulfuric acid comprises more especially treating the electrolyte so as to establish a copper concentration of less than 0.0005 weight percent, preferably less than 0.0002 weight percent, therein.

A preferred feature of the present process comprises withdrawing a portion of the sulfuric acid electrolyte from the electrolytic bath, adding commercial manganic oxide to the said Velectrolyte portion and establishing a pH-value of between 6.2 and 7.6, preferably between 6.8 and 7.2, therein, filtering the electrolyte portion so treated and recycling it to the electrolytic bath to maintain a given concentration of manganese therein.

A further preferred feature of the present process comprises subjecting the electrolyte portion to preliminary neutralization with commercial manganic oxide to irst establish a preferred pH-value of between 5.0 and 5.5 therein, adding a basic compound to then establish a pH- value of between 6.2 and 7.6, preferably between 6.8 and 7.2, therein, filtering the whole and recycling the electrolyte portion so regenerated to the electrolytic bath.

Useful basic compounds are calcium hydroxide, calcium oxide or alkali metal hydroxides.

A further preferred feature of the present process comprises adding commercial manganic oxide to a sulfuric acid manganese sulfate solution to establish a pH-value of between 5.0 and 6.5, preferably 5.5, therein, ltering the solution, flowing it through a neutral cation exchanger, wherein the cation positions are all occupied by manganese ions, and using the solution so treated as the electrolytic bath, or using it for effecting regeneration of a spent electrolytic bath.

The advantage offered by the process of the present invention resides in that the resulting manganese dioxide is practically free from lead contaminants and in that the lead cathodes have a considerably increased service life.

Crude manganese ore is the source from which undesirable copper is introduced into the electrolytic baths, which generally contain between 40 and 300y grams/liter manganese, between l0 and 120` grams/liter sulfuric acid, and have a pH-value of between 0` and 5. The copper may be present in these baths in a concentration as high as between 0.001 and 0.03 weight percent.

The following examples illustrate the process of the present invention.

EXAMPLE l A sulfuric acid manganese sulfate solution containing grams/liter MnSO4, 65 grams/liter H2804 and 100 milliliters/liter copper and having a pH-value substantially of 0.5 was preneutralized with commercial manganese oxide to first establish a pH-value of up to 5.5. Following this, a further quantity of the same manganese oxide was added to establish a pH-value of up to 6.8, and the solution was filtered. The filtrate obtained, which was an electrolyte containing 0.0002 weight percent copper, was used for regenerating a bath for the electrolytic production of manganese dioxide. Manganese dioxide, precipitated therefrom with the use of a lead cathode, was found to contain 0.10 weight percent lead. The lead cathode had a service life of more than 2,000 hours of electrolysis.

EXAMPLE 2 The procedure was the same as that described in Example 1, save that the preneutralized solution was reacted with commercial manganese oxide and calcium hydroxide until a pH-value of between 7.0 and 7.2 was found to have been established, and the solution was filtered. This treatment enabled the concentration of copper in the filtrate to be reduced down to 0.00005 percent. Manganese dioxide, precipitated therefrom with the use of a lead cathode, was found to contain 0.08 weight percent lead. The lead cathode had a service life of more than 3000 hours of electrolysis.

EXAMPLE 3 A sulfuric acid manganese sulfate solution the same as that used in Example l was neutralized with commercial manganic oxide to establish a pH-value of 5.5, and filtered. Following this, the filtrate was passed through a neutral cation exchanger, wherein the cation positions were all occupied by manganese. The electrolyte leaving the cation exchanger contained less than 0.0001 percent copper. Manganese dioxide, precipitated therefrom with the use of a lead cathode, was found to contain less than 0.08 weight percent lead, for example.

We claim:

1. A process for the electrolytic production of manganese dioxide from an electrolyte with the use of lead cathodes, the electrolyte being substantially comprised of a copper-contaminated manganese sulfate solution in sulfuric acid, which comprises establishing a concentration of copper of less than 0.0005 weight percent in the electrolyte.

2. The process as claimed in claim 1, which comprises establishing a concentration of copper of less than 0.0002 weight percent in the electrolyte.

3. The process as claimed in claim 1, which comprises establishing the concentration of copper in the electrolyte by withdrawing a portion of the sulfuric acid electrolyte from an electrolytic bath, adding commercial manganic 4 oxide to the said electrolyte portion and establishing a pH-Value of between 6.2 and 7.6 therein, filtering the electrolyte portion so treated and recycling it to the electrolytic bath to maintain a given concentration of manganese therein.

4. The process as claimed in claim 3, which comprises adding commercial manganic oxide to the said electrolyte portion and thereby establishing a pH-value of between 6.8 and 7.2 therein.

5. The process as claimed in claim 3, which comprises subjecting the electrolyte portion to preliminary neutralization to rst establish a pH-value of between 5.0 and 5.5 therein, adding a basic compound to then establish a pH-value of between 6.2 and 7.6 therein, filtering the whole and recycling the electrolyte portion so regenerated to the electrolytic bath.

6. The process a claimed in claim 5, which comprises adding a basic compound to the said preneutralized electrolyte portion to establish a pH-value of between 6.8 and 7.2 therein.

7. The process as claimed in claim 5, wherein the basic compound is a member selected from the group consisting of calcium hydroxide, calcium oxide and alkali metal hydroxides.

8. The process as claimed in claim 1, which comprises adding commercial manganic oxide to a sulfuric acid manganese sulfate solution to establish a pH-value of between 5.0 and 6.5 therein, filtering the solution, fiowing it through a neutral cation exchanger, wherein the cation positions are all occupied by manganese ions, and using the solution so treated as the electrolytic bath, or using it for effecting regeneration of a spent electrolytic bath.

9. The process a claimed in claim 8, which comprises adding commercial manganic oxide to the sulfuric acid manganese sulfate solution to establish a pH-value of 5.5 therein.

References Cited UNITED STATES PATENTS 2,741,590 4/1956 Kunin 204--96 2,867,570 l/1959 Dufour et al 204-83 3,065,155 11/1962 Welsh 204-96 X 3,455,798 7/1969 Mehne et al 204-83 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner U.S. Cl. X.R. 204-96 

